GNU C/C++/Objective-C 4.2.1 Compiler User Guide

5.6 Referring to a Type with typeof

Another way to refer to the type of an expression is with typeof.
The syntax of using of this keyword looks like sizeof, but the
construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an
expression or with a type. Here is an example with an expression:

typeof (x[0](1))

This assumes that x is an array of pointers to functions;
the type described is that of the values of the functions.

Here is an example with a typename as the argument:

typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ISO C
programs, write __typeof__ instead of typeof.
See Alternate Keywords.

A typeof-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof or typeof.

typeof is often useful in conjunction with the
statements-within-expressions feature. Here is how the two together can
be used to define a safe “maximum” macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:

The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for a and b. Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.

Some more examples of the use of typeof:

This declares y with the type of what x points to.

typeof (*x) y;

This declares y as an array of such values.

typeof (*x) y[4];

This declares y as an array of pointers to characters:

typeof (typeof (char *)[4]) y;

It is equivalent to the following traditional C declaration:

char *y[4];

To see the meaning of the declaration using typeof, and why it
might be a useful way to write, rewrite it with these macros:

#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])

Now the declaration can be rewritten this way:

array (pointer (char), 4) y;

Thus, array (pointer (char), 4) is the type of arrays of 4
pointers to char.

Compatibility Note: In addition to typeof, GCC 2 supported
a more limited extension which permitted one to write

typedef T = expr;

with the effect of declaring T to have the type of the expression
expr. This extension does not work with GCC 3 (versions between
3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which
relies on it should be rewritten to use typeof: